There is no simple answer to your question as this remains an active area of research. Embryology in general and limb development in particular teaches that specific factors such as fibroblast growth factor (FGF) in concert with the timely activation of homeobox (hox) genes leads to the ordered development of limbs. Specifically, hox genes are transcription factors (DNA binding proteins) that coordinate the expression of a whole set of genes in a given pattern, leading to the appropriate development of specific organs and body parts. Eventually, once the initial framework is built, then hormones including growth hormone, testosterone and estrogen affect gene transcription as growth occurs before and during puberty. Once the growth plates of your bones are fused, longitudinal growth ceases.